As air bubbles always want to go to the top, the UAT/air trap/last tank before pump should always be the lower than the vent, so any air will always try and go up and out a pipe into the higher fuel level.

For an example if you had a UAT on top of the main tank then any air will always try to flow into the UAT from the main tank where you dont want it. So good practice in installs to keep the UAT/header that feeds the pump as low as possible.